The present invention relates to a large-field holographic binocular visor.
The term "visor" is understood to mean a device in which a collimated light image is observed in super-imposition on a view of the external landscape. The light image corresponding to the data to be collimated is reflected at infinity by an optical collimating system. This results in absence of any effort of accommodation for the observer's eye and in substantial visual comfort. In the conventional manner, the light image is reflected toward the observer by a combination optical system. This optical system is traversed by the radiation received from the external landscape. Thus the observer such as an aircraft pilot sees a collimated light image superimposed on the landscape and corresponding, for example, to a synthetic image of navigation data.
Recent techniques in the field of high-head collimators are essentially directed to an increase in the instantaneous field of view of the collimated image as well as to the achievement of enhanced optical efficiency.
One known solution consists in making use of a spherical mirror on the axis, which also makes it possible to limit optical aberrations. In one design of this type as described in the French patent No. 2 542 459 (or U.S. Pat. No. 4,600,271), the optic axis of the spherical mirror corresponds to the normal axis of vision of the observer whose eye is placed at the center of the mirror. This mirror is combined with a semitransparent plane mirror which has the function of reflecting the optic axis from the light image generator toward the concave side of the spherical mirror which produces collimation and reflects the collimated image toward the observer. In order to increase the efficiency, a spherical holographic mirror is employed. This mirror reflects the wavelengths corresponding to the image generator which can consist of a cathode-ray tube display. The main disadvantage of this solution is that, although the circular field is enlarged, it nevertheless remains limited to values of the order of 30.degree. to 40.degree. in monocular vision. In order to obtain a larger field, it is necessary to adopt an off-axis arrangement of the spherical mirror since the partially transparent plane mirror is otherwise too close to the observer's eye. This results in optical aberrations and in difficulties in regard to practical utilization.
In accordance with another solution indicated in an article entitled "Holographic mirrors" and published in "Optical Engineering", Sep. Oct., 1985, Vol. 24, No. 5, pages 769-780, a semitransparent plane mirror is employed for reflecting the axis of the light-image generator toward the observer's eye. The corresponding radiation reflected from this mirror traverses a spherical mirror which is followed by a plane birefringent assembly perpendicular to the optic axis of the spherical mirror corresponding to the normal direction of vision. This birefringent assembly is adapted to cooperate with an upstream polarizer in order to polarize the radiation of the, light image and permits subsequent recovery of the light image after collimation by the spherical mirror. A selection of the landscape channel and of the imaging channel is thus obtained as a function of their polarization. While this solution does permit an increase in the field which can attain 60.degree. in the vertical direction and 135.degree. in the horizontal direction in binocular vision (the monocular field being 80.degree. circular), it is absolutely inefficient from the photometric point of view and presents problems when used in a real situation. In fact, transmission on the optical channel for landscape observation is below 10% and transmission on the channel for observation of the synthetic image is limited to approximately 1.6%. This results from the losses introduced in the mirrors at the time of multiple reflections and transmissions and from traversal of the polarizers. It is not possible to employ a hologram since the system is totally on the axis and the channels are not separable by holography.